This invention relates to a non-cavity backed slot antenna for a motor vehicle, and particularly to such an antenna in the roof of the vehicle. Such an antenna, being part of the vehicle body structure, is somewhat dependent on that structure for its reception characteristics, including directionality. It is desirable for any vehicle mounted antenna to be non-directional, since the vehicle does not maintain a fixed position or attitude with respect to signal sources. Generally, it is desirable to maintain a directionality standard such as no more than 15 dB between the maximum and minimum signal strength angles in the commercial FM band. This is not always attained with a slot antenna. Several methods of redesigning the slot antenna to decrease directionality, such as changing the antenna feed point, repositioning the slot or changing the geometric shape of the slot, all tend to have adverse effects on the antenna's other operating characteristics such as overall FM gain, FM bandwidth, or AM gain.
In a non-cavity backed, vehicle roof mounted, slot antenna, the pillars supporting the roof affect the vertical polarization of the radiation/reception characteristics. If the ground currents through the roof support pillars can be brought into an optimum phase relationship and amplitude relationship, the vertical polarization pattern can be made less directional.